1. Field of the Invention
This invention relates to rectifying devices and, in particular, rectifying devices involving dissimilar materials.
2. Art Background
Rectifying junctions are formed utilizing a variety of materials. For example, rectifying junctions are formed as homojunctions (two compositionally similar semiconductor materials), heterojunctions (two dissimilar semiconductor materials) and Schottky barriers (a junction formed between a semiconductor material and a material having a metallic conductivity). Irrespective of the materials employed to produce the rectifying interface, a specific set of electrical characteristics is desirable for most common applications such as photodetection and demodulation. In particular, as a first criterion, the reverse bias breakdown voltage should typically be greater than the largest voltage requiring rectification by the device. For applications such as low-voltage photodetection, this requirement makes a reverse bias breakdown greater than 5 volts generally desirable. As a second criterion, the forward bias characteristics should have an ideality factor generally equal to or smaller than 3.5. (Reverse breakdown voltage is defined in S. M. Sze, Physics of Semiconductor Devices, John Wiley & Sons, New York (1969) Chapter 2. The ideality factor, n, is defined by I.sub.F =I.sub.s e.sup.qV/nkT where I.sub.F is the forward current, I.sub.s is the saturation current, V is the voltage drop across the rectifying interface, q is the charge on an electron, k is Boltzmann's constant, and T is the temperature in degrees Kelvin.) The particular application in which a device is employed also imposes further strictures on the device characteristics. Investigations have involved a search for materials which will provide a device which satisfies the two general criteria, which satisfies the requirements set by the contemplated application, and which allows facile device fabrication. For example, many different materials have been investigated for use in photodiodes in an attempt to satisfy the ideality factor and breakdown requirements and to provide for the generation of a detectable photocurrent.
The advent of optical communications has even further increased the interest in photodiodes for use in integrated optical components and the search for suitable materials with which to fabricate these components. In these integrated components, light from the communication system is optically processed, e.g., the incoming signal consisting of a plurality of wavelengths is demultiplexed into a plurality of signals each essentially of, for example, one wavelength, and these individual signals are guided for further processing. In the same integrated component, the light is also electrically processed, e.g., the light signal which has been guided to a photodetector is converted into an electrical signal. In the fabrication of integrated optical components, it is desirable that both the optical devices (e.g., gratings and waveguides) and the signal-processing devices (e.g., rectifying junctions) be formed through a reasonably simple procedure on a substrate, e.g., a semiconductor substrate. This processing goal puts further restraints on materials contemplated for such uses. It has, therefore, been difficult to find materials that yield the appropriate electrical properties for rectifying devices, the appropriate optical properties for optical devices, e.g., gratings and waveguides, and at the same time offering the characteristics necessary for the relatively simple production of the entire integrated component.